Electrical energy is nowadays generated almost exclusively in rotating electricity producers, known as generators. By far the greatest part of the electricity generated in generators originates from turbogenerators. These run at 3000 to 3600 revolutions per minute. The rotating magnetic field is generated by windings with alternating polarity which are excited by direct current. The windings emerge from the longitudinal grooves of the rotor at the body ends and form the end winding which is secured against centrifugal force. This prevents the copper windings (rotor end winding) that meet at the end from flying out as a result of centrifugal forces.
The rotor cap, in contrast to the rotor itself, is generally made of a high-alloyed non-magnetizable steel and, together with the slot wedges at the surface of the rotor, forms an electrically conductive cage which is known as the damper winding. The damper winding serves to reduce shock loads (pole wheel oscillations) and the heating of the rotor in the case of a load imbalance. In the case of symmetrical load, no alternating fields and thus also no eddy currents occur in the rotor, which is forged from solid steel. However, as a result of a load imbalance, a magnetic rotating field occurs in the steel core of the rotor, and this can result in eddy currents and in impermissible heating and, in extreme cases, in the destruction of the turbogenerator. This needs to be ruled out for the projected service life. These functions are taken on by the rotor caps. They are the most highly loaded components of the turbogenerator. The rotor caps are in most cases shrunk in an overhung manner onto the body ends and are secured against rotational and axial movements by a kind of bayonet connection, for example. On the side remote from the bodies, a support ring is in many cases shrunk into the rotor cap, said support ring absorbing the forces of the end windings that act in the longitudinal direction.
The development of ever larger power units is resulting in increasing stress on the rotor caps. Therefore, the structural and material design of the rotor caps is subject to the highest requirements. It is also necessary, with increasing output, in particular in turbogenerators, to step up cooling. It is known that air is predominantly used as cooling medium in generators in the range up to about 300 MVA. At greater outputs, hydrogen is also used as cooling fluid. The cooling fluid and the manner of cooling have consequences for the structural configuration of the generator, in particular also the coils in the stator and in the rotor. As far as the manner of cooling is concerned, a distinction is made between indirect and direct conductor cooling. Furthermore, it is known practice to cool generators using suction ventilation or pressure ventilation. In the case of suction ventilation, the cooling fluid is conducted in ducts from the main fan to the cooler. From there, the cooling medium flows without heating up as a result of fan losses and compression to the machine parts to be cooled, in particular also to the rotor coils. For flow generation, axial fans are generally used for cost reasons both in pressure ventilation and in suction ventilation in the case of air cooling. In the case of hydrogen cooling, radial fans and multistage axial fans are also used.
Hitherto, the rotor caps have generally been produced from metal or an alloy.
For example, EP 1628382 B1 discloses a structure in which the rotor caps or cap rings (both of these terms are used synonymously in the present document) are made of alloyed metal. Although optimum mechanical properties are realized in this case, both production times and production costs are very high and there is no design for cooling.